LT1881/LT1882

9

APPLICATIO S I FOR ATIO

The LT1881 dual and LT1882 quad op amps feature

exceptional input precision with rail-to-rail output swing.

The amplifiers are similar to the LT1884 and LT1885

devices. The LT1881 and LT1882 offer superior capacitive

load driving capabilities over the LT1884 and LT1885 in

low voltage gain configurations. Offset voltages are trimmed

to less than 50

µV and input bias currents are less than

200pA on the “A” grade devices. Obtaining beneficial

advantage of these precision input characteristics de-

pends upon proper applications circuit design and board

layout.

Preserving Input Precision

Preserving the input voltage accuracy of the LT1881/

LT1882 requires that the applications circuit and PC board

layout do not introduce errors comparable to or greater

than the 30

µV offset. Temperature differentials across the

input connections can generate thermocouple voltages of

10’s of microvolts. PC board layouts should keep connec-

tions to the amplifier’s input pins close together and away

from heat dissipating components. Air currents across the

board can also generate temperature differentials.

The extremely low input bias currents, 150pA, allow high

accuracy to be maintained with high impedance sources

and feedback networks. The LT1881/LT1882’s low input

bias currents are obtained by using a cancellation circuit

the input resistances in each input lead, as is commonly

recommended with most amplifiers. The impedance at

either input should be kept as small as possible to mini-

mize total circuit error.

PC board layout is important to insure that leakage cur-

precision, high impedance circuits, the input pins should

be surrounded by a guard ring of PC board interconnect,

with the guard driven to the same common mode voltage

as the amplifier inputs.

Input Common Mode Range

The LT1881 and LT1882 outputs are able to swing nearly

to each power supply rail, but the input stage is limited to

this common mode range will cause the gain to drop to

zero; however, no gain reversal will occur.

Input Protection

The inverting and noninverting input pins of the LT1881

and LT1882 have limited on-chip protection. ESD protec-

tion is provided to prevent damage during handling. The

input transistors have voltage clamping and limiting resis-

tors to protect against input differentials up to 10V. Short

transients above this level will also be tolerated. If the input

pins can see a sustained differential voltage above 10V,

external limiting resistors should be used to prevent

damage to the amplifier. A 1k resistor in each input lead

will provide protection against a 30V differential voltage.

Capacitive Loads

The LT1881 and LT1882 can drive capacitive loads up to

1000pF in unity-gain. The capacitive load driving in-

creases as the amplifier is used in higher gain configura-

tions. Capacitive load driving may be increased by

decoupling the capacitance from the output with a small

resistance.